Methods for detecting submerged objects such as mines may be categorized as either passive or active systems. Passive systems emit no energy, but instead sense disturbances in the underwater environment created by the presence of a submerged object. Active systems, on the other hand, rely on the sensing of emitted energy either reflected or radiated by the object to be detected. One example of the latter type of system is a conventional sonar device wherein a directional beam of acoustic energy periodically radiates from a scanning transducer which in turn may operate as a receiver to detect echoes reflected from an object within the path of propagation. Although satisfactory for many uses, sonar devices have several inherent limitations. Nearby objects can cause echoes and these may obscure the echo of the object to be detected. A rocky bottom will produce many echoes, thus making efficient detections of objects on the bottom almost impossible. In addition, objects whose specific acoustic impedance is similar to that of water will produce only a small echo or no echo at all if their specific acoustic impedance is the same as that of water.
Another type of active system includes systems which set up electric or magnetic fields and measure variations therein. Magnetic objects will cause a variation in the magnetic field due to their being magnetized by the applied magnetic field while non-metallic objects immersed in sea water will cause a variation in the electromagnetic field by creating a "hole" in the conductivity pattern of the sea water.
In general, the variations in the applied electromagnetic field created by these objects is small and it has, in the past, been difficult to find sensors that could detect these perturbations. Once found, an object must be identified so that one can tell whether it is a hostile threat. Additionally, there may be hundreds of mines within a given region such as a mined surf zone which must be neutralized. Then, after finding a mine either floating, proud or buried, its effectiveness must be destroyed, either by removal or by destruction.
Accordingly, it is an object of the present invention to detect and identify both metallic and non-metallic underwater objects.
It is also an object of the invention to detect and identify bottom, proud and buried mines.
A further object of the invention is to determine the location of underwater objects such as mines with respect to an electromagnetic grid.
Another object of the invention is to provide a navigational grid which can be used by landing craft and other kinds of boats and vessels.
Yet another object of the invention is to generate an electromagnetic field having a navigational grid therein.
It is yet another object to provide a detonating device and means for detonating mines by command signal or at a specific time so as to provide a single detonation event for all mines within a specific channel.